The umbrella regime for the protection and preservation of the marine environment is composed, first of all, of principles of customary international law. These principles are legally binding and universally applicable regardless of their codification within a treaty. However, only a few of them apply to the marine environment and their contribution to ocean preservation is quite limited.
Customary law places States under a general duty to “prevent” damage to the marine environment (the preventive principle) and to use “due diligence” in conducting harmful activities under their national jurisdiction or control. In both cases, however, States are required to take appropriate measures for which they are capable, financially and technologically. In many cases (e.g., eutrophication from agricultural practices) the costs of prevention may be too high to require preventive action. A high degree of scientific certainty and predictability of the harmful effects of human activities, moreover, are essential conditions for the adoption of preventive measures.
Moreover, States are under a general duty not to cause “serious or significant” damage to the environment of other states or areas beyond national jurisdiction, such as the high seas (sic utere tuo ut alienum non laedas) and not to “unreasonably” interfere with the traditional freedoms of other states in this area (the reasonable use principle).
Presumably, on the basis of these principles, States may not allow their nationals to conduct activities (e.g., to discharge hazardous substances or destructive fishing practices) in a manner that could cause harm or be prejudicial to the interests of other States. Both principles, however, only apply to transboundary situations. They do not establish a positive legal duty to protect the marine environment within waters under national sovereignty or jurisdiction, but only a responsibility to compensate environmental damage.
Furthermore, States are under a general duty of cooperation by means of information exchange, consultation and notification. This duty, however, only emerges in the case of transboundary pollution and/or in emergency situations (the principle of good neighbourliness and international cooperation).
Generally speaking, according to the principles of customary international law not all interferences, harm or damage must be avoided or prevented, but only those which are “unreasonable”, “serious”, “appreciable” or “significant” and States have a great deal of discretion in determining these thresholds. These principles, therefore, are too general and too broadly formulated to require States to take concrete action to protect the marine environment.
All the principles discussed so far were established when international environmental law was still at its very early stage. In the course of the 1980s, in the run-up to UNCED, new objectives, principles and concepts, such as sustainable development, the precautionary principle and the ecosystem approach, have emerged and, were endorsed in Agenda 21. Their legal status, however, is still unclear. Nevertheless, most of these emerging objectives and principles (e.g., sustainable development and the precautionary principle) have been codified in the EC Treaty and, therefore, have legally binding status within the EC legal system.
